In various mechanical assemblies, it is known to utilize steel cable to connect a working part to the actuating part, e.g. in throttle, detent, clutch and brake assemblies. A tube or sleeve within which the cable slides is provided to guide the cable. It is essential that this tube should have adequate impact resistance and resistance to deformation under load coupled with low friction and high wear resistance as the cable slides within the tube.
One type of composition which has been proposed for use in bearings of various types is a blend of nylon with a minor proportion by weight of polytetrafluoroethylene ("PTFE"). Pertinent patent disclosures are the following.
U.S. Pat. No. 3,005,795 of Oct. 24, 1961 to Busse et al. This patent points out difficulties in fabricating high molecular weight organic polymers such as polyamides and others by extrusion, because of the rapid change of viscosity with temperature in such polymers. The patent proposes blends of such polymers with, as modifier, polytetrafluoroethylene resin in the form of finely divided microfibrous and submicrofibrous particles having diameters ranging from about 100 A up to about 2 microns, i.e. from 0.01-2 microns. The utility disclosed in the only Example using nylon (Ex. 6, nylon-6,6) is "for extrusion as a coating on wire and as film or thin sheeting." It is stated that treatment of PTFE by heating above 330.degree. C. for some time, or by irradiation by high energy electron beam renders the polytetrafluoroethylene not suitable for use in the practice of the subject invention, because the PTFE thus treated no longer tends to form ultra-fine fibrils.
U.S. Pat. No. 3,287,288 of Nov. 22, 1966 to Reiling. The patent relates to compositions of powdered tetrafluoroethylene resin with a thermosetting or a thermoplastic resin to produce a ball-joint socket, sleeve bearing or other type bearing having low surface friction characteristic and considerable resistance to deformation under load. The PTFE is initially degraded by exposure at elevated temperature for a period of about 2 hours, whereby its tendency to agglomerate is ameliorated. Use of "nylon type plastics" is disclosed.
U.S. Pat. No. 3,908,038 of Sept. 23, 1975 to Nienart et al. This patent relates to thermoplastic compositions containing a polyamide such as polycaproamide (i.e. nylon-6), finely divided PTFE, and an inert filler from the group consisting of silica and magnesium oxide to provide increased creep and wear resistance for production of bearing surfaces. In the examples, the compositions are injection molded or compression molded. The subject compositions contain 40 to 60% PTFE along with correspondingly 60 to 40% of polyamide and additionally, about 2 to 11% by weight of the composition of the inert filler.
Also, of interest in the prior art is a literature article "Plastics World" of October 1976, pp. 46 et. seq. relating to compositions from nylon-6,6, i.e. poly(hexamethylenediamine adipamide) and PTFE of "low molecular weight". The compositions, especially those containing a filler such as glass fiber, are used in producing bearings.
An important consideration in producing plastic tubing, such as the above-noted sleeve bearings for steel cable, is that the plastic used should be extrudable to commercial quality tubing at commercial rates. In general, the nylon/PTFE blends of the prior art, even if extrudable to films and sheets, are not extrudable to tubing because production of tubing requires high dimensional stability in the fused state, to maintain the proper inside and outside diameters of the tubing until the extruded tubing has cooled sufficiently to hold its shape.